利用java自带的排序。先把非字母的去掉，最后再拼回来

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        while (sc.hasNextLine()) {
            String string = sc.nextLine();
            List<Character> list = new ArrayList<>();
            for (char c : string.toCharArray()) {
                if (Character.isLetter(c)) {
                    list.add(c);
                }
            }

            list.sort(new Comparator<Character>() {
                @Override
                public int compare(Character o1, Character o2) {
                    return Character.toLowerCase(o1) - Character.toLowerCase(o2);
                }
            });

            StringBuilder stringBuilder = new StringBuilder();
            int count = 0;
            for (char c : string.toCharArray()) {
                if (Character.isLetter(c)) {
                    stringBuilder.append(list.get(count));
                    count++;
                } else {
                    stringBuilder.append(c);
                }
            }

            System.out.println(stringBuilder);
        }

    }
}